1. Field of the Invention
The present invention relates to a method of preparing a compound semiconductor crystal, and more particularly, it relates to a compound semiconductor crystal which is employed as a substrate for fabricating optical devices such as LEDs or laser diodes, or ICs, and a method of preparing the same.
2. Description of the Background Art
It is known in the art that compound semiconductor crystals of n-type GaAs or the like which are to be employed as substrates, for example, are prepared by various industrial methods such as a horizontal Bridgman method (HB method), a liquid encapsulated Czochralski pulling method (LEC method) which is disclosed in Japanese Patent Laying-Open No. 3-57079 (1991), a vertical Bridgman method (VB method) and a vertical temperature gradient freeze method (VGF method).
Among these methods, a vertical boat method, i.e. the VB or VGF method, is regarded as a promising method which can prepare a high-quality crystal having a lower defect density as compared with the other above mentioned methods at a low cost.
In relation to such a vertical boat method, i.e. the VB or VGF method, Japanese Patent Laying-Open No. 4-367589 (1992), for example, discloses a method of preparing a GaAs crystal while using a B.sub.2 O.sub.3 encapsulant.
In preparing of a compound semiconductor crystal by the vertical boat method, first a seed crystal is arranged on a bottom portion of a crucible and a raw material solid is arranged thereon. Then, the raw material solid is entirely melted while an upper portion of the seed crystal is also melted. Then, the raw material melt as formed is cooled so as to be solidified upwardly from the seed crystal, thereby growing a single crystal.
However, a compound semiconductor substrate which is obtained by the aforementioned vertical boat method is still to be improved in consideration of its dislocation density. This is because the dislocation density must be further reduced if such a compound semiconductor crystal substrate is to be applied to fabrication of optical devices or ICs, due to influence exerted on the yield of elements which are obtained from a single substrate and the life time of the devices as formed.
In general, it is impossible to stably obtain an Si doped GaAs single crystal having a prescribed carrier concentration by the aforementioned vertical boat method by introducing Si into the crystal to attain a constant Si concentration. This is because an activation factor is not controlled in general. In the case of an Si concentration of 5.times.10.sup.17 atoms/cm.sup.3, for example, the carrier concentration is 0.5.times.10.sup.17 /cm.sup.3 if the activation factor is 10%, while the former is 4.times.10.sup.17 /cm.sup.3 if the latter is 80%.
Consequently, a number of defective elements result from industrial preparation of compound semiconductor crystals having prescribed carrier concentrations, and hence the preparation cost is disadvantageously increased.